The purpose of this study is to determine the extent to which impairment of the central nervous system will be prevented in hypothyroid infants identified by newborn screening programs. Hypothyroidism as defined for this study is characterized by confirmed abnormally low circulating concentrations of thyroxine (T4) and elevated levels of thyrotropin (TSH). The rationale for this study is based on previous data which suggest that the earlier the treatment the better will be the eventual intelligence quotient (I.O.). Infants with congenital hypothyroidism discovered by the New England Regional Hypothyroidism Screening Program will be managed by a standard protocol of treatment and psychometric clinical assessments. The relationships of age at onset of treatment and a variety of other variables to eventual I.O. will be established. Similar measurements will be made on control groups composed of (1) normal siblings, (2) hypothyroid children diagnosed clinically since 1970, (3) children with persistently low T4 levels but with normal amounts of throxine binding globulin (TBG) and TSH. Intellectual status will be determined by standard psychometric measurements and by school performance (Bayley Scales, Stanford Binet, WPSSI, and neuropsychologic examination by the Reitan procedures).